Obama's visit to Seoul comes as North Korea has threatened to conduct its fourth nuclear test, leading Obama to raise the possibility of further sanctions.

"The commitment that the United States of America has made to the security of the Republic of Korea only grows stronger in the face of aggression," Obama said in a speech to some of the 28,000 American service members stationed in South Korea to keep watch on its northern neighbor. "Our alliance does not waiver with each bout of their attention seeking. It just gains the support of the rest of the world."

The website 38 North, which closely monitors North Korea, said commercial satellite imagery from Wednesday showed increased movement of vehicles and materials near what are believed to be the entrances to two completed tunnels at Punggye-ri nuclear test site. The movements could be preparations for an underground atomic explosion, although predicting underground tests is notoriously difficult.

Obama ridiculed North Korea's attempt to show force. "Anybody can make threats," he said. "Anyone can move an army. Anyone can show off a missile. That doesn't make you strong."

He said real strength comes from having an open participatory democracy, open markets and a society free to speak out against its government.

"We don't use our military might to impose these things on others, but we will not hesitate to use our military might to defend our allies and our way of life," Obama said to cheers from the uniformed troops who filled a field house at Yongsan Garrison, headquarters for U.S. forces in South Korea.

Obama's 10-minute speech followed a rare joint defense briefing with South Korean President Park Geun-hye that focused on efforts to counter the North's nuclear ambitions. U.S. Army Gen. Curtis Scaparrotti, the commander of the joint U.S.-South Korea command, told the two presidents that his team "works together every day to make sure that we defend the Republic of Korea and that we deter North Korea."

Following his remarks, Obama left for Malaysia, the third stop on his four-country Asia swing. He opened his trip in Japan and planned to close his weeklong swing in the Philippines early next week. The travel fatigue showed as the president slowly walked up the steps of Air Force One for the Malaysia flight, instead of his usual brisk jog onto the plane.

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